Sporulation normally starts when the growth medium runs out of rapidly metabolizable carbon or nitrogen sources, phosphate or amino acids. The ultimate metabolite controlling this differentiation process was shwon to GTP, whose partial deficiency initates sporulation. Partial amino acid deficiency also causes sporulation because it invokes the stringent response, including the synthesis of ppGpp, and thereby causes a decrease of GTP. The phenomenon of sporulation initiation was analyzed under conditions which normal sporulation was prevented by the presence of excess glucose, ammonium ions and phosphate. In this medium, several inhibitors of purine nucleotide synthesis and purine analogs induced sporulation, whereas inhibitors of other biochemical reactions were ineffective.